This invention relates to permanent magnet electrical generators which rotate at high speeds. Such generators are used for general purposes and in various advanced technologies including hybrid-electric vehicles, variable speed constant frequency engine-generator sets and energy-storage flywheels.
An object of the present invention is to provide an electrical generator which produces an output which is readily and inexpensively convertible into a fully controllable AC output signal.
Rotary electrical generators produce raw alternating current (AC) signals, the frequencies of which are proportional to the generator rotational speeds. For example, a two pole generator running at 3,600 rpm will have a 60 Hz output. However, many generators are driven by devices such as gas turbine engines and energy-storage flywheels which usually rotate at a high speed, 10,000 rpm for example, which is far too fast to produce a raw output signal of 60 Hz. Very high frequency generator AC is unsuitable for most practical purposes. Thus, it has been a practice either to interpose a mechanical speed reducer between the driving device and the generator, or to utilize an electrical circuit which converts the high frequency AC generator output first to direct current (DC) and then to a lower frequency AC.
Speed reducing mechanisms have included reduction gearing, belts, pulleys, and other devices, all of which add to the cost of the system, require maintenance and repairs, and inherently reduce the overall efficiency of the system.
Electrical circuits for processing high frequency generator output signals often use pulse width modulated (PWM) amplifiers. PWM amplifiers are expensive, they generate electrical noise, and they require large heat sinks to dissipate the substantial quantity of heat which they generate. Due to their choppy outputs, they also require large banks of electrical filters such as electrolytic capacitors which are expensive and prone to failure.
A prior art PWM circuit amplifier for processing the electrical output of a high speed generator 1 is shown in FIG. 1. It includes a full wave rectifier 3 which has four diodes and converts the high frequency raw AC signal to a pulsing DC signal and an inverter circuit 5 which is an H-bridge formed of four insulated gate bipolar transistors (IGBTs) which are turned on and off according to a selected pattern to produce an AC signal which has the desired frequency. This signal is fed through a filter network 7 and a transformer 9 to an electrical load. The IGBTs are turned on and off when there is a high voltage across them. This results in power losses and it produces undesired electrical noise.
In systems which utilize the invention, the generator output signal waveform is affected by using an actuator which gives the generator output coils an oscillating motion which is independent of the normal rotary motion of the generator. The oscillation motion moves the generator output coils toward and away from a null flux position at which the generator produces near zero output voltage. The frequency of this oscillating motion is equal to the desired and selected frequency of the final alternating current output of the apparatus. This frequency is referred to in the claims as the "mechanical oscillation frequency." To simplify the conversion of the raw generator output to AC of a useful desired frequency, the actuator is driven by alternating current to produce a mechanical oscillation which has the desired frequency. The power required to drive the actuator is relatively low, preferably less than one percent of the output power of the generator.
Due to the waveform of the output signal from the generator of the present invention, and the relationship between this waveform and the frequency of the signal which energizes the actuator which moves the generator output coils, any electronic circuitry for processing the generator output can be simplified. The circuitry can use electronic switches such as thyristors (often referred to as SCRs) which are commercially available in high power ratings (greater than 1 megawatt, for example) and are less expensive than electronic switches such as IGBTs. Because the SCRs are switched on and off when the voltages are at or near zero, power losses are low, and electrical noise is minimized.
The displacement responsive generator (DRG) according to the present invention, with its output processing circuitry, is believed to be substant ally less bulky, less expensive, more efficient, and more reliable than existing systems which utilize mechanical transmissions or PWM amplifier technology.